1. Field of Invention
This invention relates to the Global Positioning System (GPS). Specifically, the present invention relates to low phase error antennas for receiving GPS signals.
2. Description of the Related Art
The Global Positioning System is used in a variety of demanding applications ranging from geological surveys, to military positioning applications. Such applications require accurate antennas to precisely determine distances and positions with sub-millimeter accuracy.
The Global Positioning System includes a constellation of satellites equipped with GPS transmitters. A ground receiver receives signals from the satellites. By measuring signal travel time from the satellites to the phase center of the ground receiver's antenna, the position of the ground receiver may be determined. The phase center of the antenna corresponds to the point at which the antenna appears to receive a spherical wavefront. The phase center may be different than the physical center of the antenna.
Often, the phase center of the antenna does not correspond the physical center of the antenna due to multipath errors and/or phase errors. Typical GPS antennas are either dual frequency patch antennas or cross dipole antennas which are particularly prone to phase and multipath errors. Multipath errors occur when signals transmitted from the GPS satellites reflect off hills or objects and combine. The combined signal is received by the ground receiver and results in an effective electrical position that erroneously moves with satellite transmit location. An antenna with a receive pattern that extends well below horizontal may more readily detect such combined reflected signals. An antenna with such a receive pattern is said to have a large backlobe and is more susceptible to multipath problems.
Phase errors are inherent in certain antenna element designs such as patch antenna designs. Other phase errors occur due to manufacturing tolerance such as in cross dipole designs. Phase errors cause the phase center of a stationary ground antenna to move with satellite position. The effective phase center of patch antennas and cross dipole antennas often vary with GPS satellite position due to antenna structure and manufacturing error respectively.
To reduce multipath errors choke slot groundplanes were developed. Choke slots are highly reactive devices at the design frequency which when installed on a GPS antenna reduce antenna surface currents and re-radiation. The reduced surface currents may result in a decreased antenna backlobe and reduced multipath errors. The antenna is said to have improved multipath rejection. GPS antennas that employ choke slots are often large and expensive as a result of structural limitations.
To reduce phase errors associated with existing GPS antennas a method known as observation differencing was developed. Observation differencing involves canceling phase errors through the introduction of compensation variables. This method requires antennas in the GPS system to be of the same make and model. The method relies on the assumption that antennas of the same make and model behave similarly. The lack of consistency between such antennas limits the effectiveness of observation differencing in canceling phase errors. This lack of consistency is partially due to manufacturing inconsistencies due to difficult tooling procedures.
Hence, a need exists in the art for a cost effective, compact antenna that minimizes phase and multipath errors. There is a further need for antenna that provides for tooling procedures that result in antennas with similar and consistent performance.